1. CToA Protocol Analysis
Month Year
doc.: IEEE yy/xxxxr0
CToA Protocol Analysis
Date:
Jonathan Segev, Intel

2. This presentation analyzes two aspects of the CToA protocol:
Abstract
This presentation analyzes two aspects of the CToA protocol:
Positioning accuracy:
Theoretical measurement model error analysis
Geometric-theoretical error analysis
Performance in presence of biased measurements
Unmanaged operation of the CToA network:
Requires no scheduling protocol (either among bSTAs or clients RX/TX) .

3. Recap: CToA Network Entities
July 30, 2013
Presentation Title
Recap: CToA Network Entities
CToA Broadcasting Station = bSTA
CToA client Station = cSTA
TAG
TX (broadcast) + RX of measurement messages
Asynchronous
RX – only
TX – only (broadcast, asynchronous)
Copyright © 2013 Intel. All rights reserved.

4. COMPARATIVE POSITIONING ACCURACY analysis

5. Comparative Performance Analysis
Theoretical measurement model error analysis
Geometric-theoretical error analysis
Performance in presence of biased measurements
Most of the details are presented in [1]
Notation:

6. CToA Measurements Model
To obtain a “1st Fix” the cSTA has to jointly solve the position and clock offsets vector.

7. CToA with Kalman Filter Converged
With KF converged, all the clock offsets are known up to a residual estimation error:
The noise level associated with the measurement may be modeled as:
denotes the std of the KF’s residual estimation error of the clock offsets
CToA accuracy in this mode is trilateration/RTT-like geometry equivalent.

8. RDToA The observed time difference measurement is defined as [2]:
The noise level associated with the measurement may be modeled as:
Multilateration (hyperbolic) geometry

9. Geometric-Theoretical Positioning Accuracy in an Indoor Environment
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doc.: IEEE yy/xxxxr0
Geometric-Theoretical Positioning Accuracy in an Indoor Environment
“Heat-map” depicts 95% positioning errors evaluated using theoretical 2D error-covariance (evaluated using Cramér Rao lower bound /CRLB).
Method details can be found in [1]
Jonathan Segev, Intel

10. Positioning Accuracy with Biased Measurements
For a given network of N bSTAs, RDToA can measure up to:
non-recurring, differential ranges.
Assuming there are N bSTAs, then while with CToA only 1 measurement becomes biased, with RDToA (N-1) measurements are affected.
Namely, all the differential range measurements involving the “biased” bSTA.

11. Positioning Accuracy with Biased Measurements – cont’d
Client estimates position using 4 closet bSTA
Client has NLoS with 1 bSTA => measurement bias (5m NLoS bias)

12. CToA Unmanaged Operation

13. General Assumptions for CToA Network
In worst-case scenario the bSTA may be implemented as a standard AP
Each AP has a “native” channel – used for data exchange with associated STAs.
Need to be always available, and if not, announce it in advance.
Optionally, bSTAs act as dedicated “FTM responder-like” STAs.
CToA RX events are asynchronous
Every bSTA broadcasts measurement 3-5Hz
Each bSTA is assumed to first scan the medium to detect prior CToA broadcast activity.

14. Potential Protocol: “RX Collection of CToA Measurement Messages”
The AP learns the periodicity of CToA measurement messages broadcast by other AP/bSTAs.
The AP announces to its associated STAs on a “non-availability” period
The AP switches to neighbor bSTA channels and receives (“collects”) their CToA measurement messages.
Since the bSTAs are asynchronous, the AP may need to switch many times every second => high overhead and low availability on its native channel.

15. The Solution: “TX Sweep”
Instead of running “RX collection” use a “TX distribution” approach:
“Bundle” all off-channel transactions into a single, contiguous window in time.
Less than 1ms out of a 200ms CToA meas. messages broadcast interval
Off-channel activity is independently scheduled by each AP.
Enables the AP to announce its off-channel timing at low frequency (3-5Hz or even lower).

16. Off-Channel Timing Optimization
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doc.: IEEE yy/xxxxr0
Off-Channel Timing Optimization
bSTA TX CToA meas. messages on every active channel, RX CToA meas. messages only on its native channel
The bSTA broadcasts two types of CToA measurements messages:
“Short” (only w/ ToD, w/o LMR) are broadcast on every channel, other than “native AP channel”
Enables neighbor bSTAs to measure ToA
“Long” (w/ ToD & LMR) only on “native AP channel”
LMR is only relevant for the cSTAs to estimate their position.
bSTA listens for neighbor bSTA broadcasts (and regular uplink traffic) on its native channel.
If a particular channel is busy, it may be skipped for the current window, and attempted again next time.
A cSTA collecting CToA meas. messages should be able to collect LMR from all bSTAs within ~ ms (depending on the broadcast rate).
Jonathan Segev, Intel

17. bSTA “TX Sweep” 2 bSTAs broadcast Month Year
doc.: IEEE yy/xxxxr0
bSTA “TX Sweep”
2 bSTAs broadcast
Jonathan Segev, Intel

18. General Assumptions for CToA Client NW-Centric (E.g., Asset Tracking)
bSTA protocol in NW-centric mode is identical to client-centric
Enables bSTA to concurrently support client-centric & NW-centric applications
Client may be unassociated with any AP.
Client broadcasting protocol is similar to bSTA broadcast protocol
Provided that the client (e-Tag):
Broadcasts the ToD of its meas. messages.
Broadcasts meas. messages frequently enough (according to its XTAL stability) – 3-5Hz.
Then, each individual meas. message is captured by a different bSTA at a different time, and yet the positioning server would still be able to estimate the e-Tag’s position.
No scheduling between the bSTAs is required.
TX-sweep duty cycle depends on application requirements.
Could vary from seconds to minutes (or more).
TAG
A CToA e-Tag is like a “mobile” bSTA (just never broadcasting LMR)

19. CToA Client/eTag in NW-Centric Mode – TX Sweep

20. SUMMARY & conclusions

21. Conclusions
Theoretical and practical performance of CToA is similar to RTT:
Once KF is converged, clock offsets are known up to a residual error
CToA is more robust to biased measurements than RDToA.
CToA can be implemented over an unmanaged, asynchronous network of bSTAs, which may be implemented as part of a standard AP.
No scheduling protocol between bSTAs is required.
Similar scheme is used for both client and NW-mode, where clients operating in NW-mode act like “mobile bSTA”.

22. References
[1] IEEE R0 CToA Whitepaper [2] IEEE – Annex P.3

23. Thank you for your attention!

24. BACKUP SLIDES

25. 802.11az NDPA Data – Event Log Mandatory Data
bSTA event log:
The table contains logging of events (TX or RX) from the past X seconds.
ToD / ToA field includes a 48bit counter of the measured ToD/ToA time-stamp in [ps]
Format similar to REVmc FTM
Ensures a low-rate wrap-around (once every s)
Per each transmitted packet:
Packet ID (PID)
NDP time of departure (ToD)
TX MAC (own MAC)
Per each received packet:
NDP time of arrival (ToA)
TX MAC
RX MAC (own MAC)
PID
(TX or RX) MAC
ToD / ToA
{16 bit}
{48 bit}

26. CToA MAC-Level Overhead Analysis
Parameter
Value
#bSTA RX direct "A" 4
#Neighbor bSTA published "N"
[Broadcasts/sec] per bSTA "B" 3
Event log window [sec] "W" 1
ToD entries (self broadcast) "M1"
ToA + ToD entries (RX direct) "M2" 24
#Entries in event log table (#measurements) 27
Data per broadcast [Kbit] "D"
5.3
Data rate consumed per bSTA [Kbit/s]
16.0
An event log table entry = 1 measurement
#Entries in event log table: M1+M2
M1 = (N+1)*W*B
M2= A*(N+1)*W*B
Data per broadcast: D = (M1*112bit+M2*208bit)/1000 [Kbit]
Data rate consumed per bSTA = D*B
PID
TX/RX MAC
ToD / ToA
Total
ToD -only
16 bit
48 bit
112 bit
ToD+ToA
2*48 bit
208 bit

27. CToA PHY Level Overhead
Month Year
doc.: IEEE yy/xxxxr0
CToA PHY Level Overhead
Parameter
Duration
Repetitions
Total
NDPA
188µs* 1
SIFS
16µs
L-STF
8µs 2
L-LTF
L-SIG
4µs
VHT-SIG-A
VHT-STF
VHT-LTF 4+
272µs
* Assuming 40MHz/117 subcarriers, BSPK modulation, incl. 168bit of fixed NDPA overhead
+ Ntx = 4
NDPA Frame format
NDP Frame format
Jonathan Segev, Intel